The use of gamma ray detectors in general, and positron emission tomography or PET detectors in particular, is growing in the field of medical imaging. In PET imaging, a radiopharmaceutical agent is introduced into an object to be imaged via injection, inhalation, or ingestion. After administration of the radiopharmaceutical, the physical and bio-molecular properties of the agent will cause it to concentrate at specific locations in the human body. The actual spatial distribution of the agent, the intensity of the region of accumulation of the agent, and the kinetics of the process from administration to its eventual elimination are all factors that may have clinical significance. During this process, a positron emitter attached to the radiopharmaceutical agent will emit positrons according to the physical properties of the isotope, such as half-life, branching ratio, etc.
The radionuclide emits positrons, and when an emitted positron collides with an electron, an annihilation event occurs, wherein the positron and electron are destroyed. Most of the time, an annihilation event produces two gamma rays (at 511 keV) traveling at substantially 180 degrees apart.
By detecting the two gamma rays, and drawing a line between their locations, i.e., the line-of-response (LOR), one can retrieve the likely location of the original disintegration. While this process will only identify a line of possible interaction, by accumulating a large number of those lines, and through a tomographic reconstruction process, the original distribution can be estimated. In addition to the location of the two scintillation events, if accurate timing (within few hundred picoseconds) is available, a time-of-flight (TOF) calculation can add more information regarding the likely position of the event along the line. Limitations in the timing resolution of the scanner will determine the accuracy of the positioning along this line.
For a clinical, whole-body PET scanner, the imaging field-of-view (FOV) is a cylindrical volume, with a centered circular region in the transverse plane whose diameter is less than the bore size of the scanner, and the same axial length as the PET scanner. For conventional PET scanners, the traverse FOV, which is the diameter of the circular region in the transverse plane, is typically one of two possible values, e.g., 256 mm for a brain scan, and approximately 576-700 mm for a whole-body scan. Moreover, for each of these two possible PET FOVs, the same fixed coincidence window (in the range of 4-6 ns) is used.
In contrast, as shown in Table 1, conventional CT systems support multiple FOVs.
TABLE 1CT FOVSMLLLXLXXLDiameter (mm)240320400550700850
A problem with conventional PET scanners is that the predetermined whole-body FOV is inadequate for the entire range of patients having different sex, age, body type, and size. Moreover, since an associated CT scan is typically used to obtain the anatomical information of a patient, the different FOV settings used for CT should be adopted for PET. However, using the same fixed coincidence window for different FOVs in PET is inadequate since a large coincidence window for a small FOV increases random coincidences in the prompt data, which degrades the image quality and quantification.